The gene will produce a pheromone called E-beta-farnesene that is normally emitted by aphids when they are threatened by something.

When aphids smell it, they fly away.

"Also, the natural enemies of aphids - ladybirds, lacewings and a particular parasitic wasp - when they smell this smell, they're attracted," said Prof Huw Junes, one of the study team who signed the open letter.

"So it's potentially got an advantage in the UK and other western nations because it'll prevent the need to spray insecticide - and [in the developing world where] farmers don't have access to insecticide, they'd have that packaged up in the seed."

However Lucy Harrap from Take the Flour Back doubted the crop's environmental credentials.

Aphids are a major threat to cultivation in the UK and other countries

"So far, the evidence doesn't indicate that GM fields need less pesticide - in fact they tend to need more," she said.

"The other thing is that they're using an antibiotic resistance gene as a marker in this trial, and in many parts of the EU that's considered quite outdated science now because you can get gene transfer into bacteria and so on."

The group's publicity material suggests the crop contains a cow gene. Its logo is a cow's head with a body in the shape of a loaf.

The gene in question - a promoter gene, which switches on other genes - is a synthetic variant of one found in many organisms, including wheat itself.

The researchers explained that they chose a variant closer to the cow version than the wheat one in order to prevent other genes in the wheat recognising its activity and regulating it.

E-beta-farnesene itself is produced naturally by a number of plants including peppermint and potatoes.

Security concerns

Most biotech crops grown across the world are proprietary to big commercial companies such as Monsanto and Syngenta.

In contrast, the Rothamsted letter pledges their results "will not be patented and will not be owned by any private company.

"If our wheat proves to be beneficial we want it to be available to farmers around the world at minimum cost," they write.

They are inviting campaigners in for a discussion.

"You have described genetically modified crops as 'not properly tested'," they write. "Yet when tests are carried out you are planning to destroy them before any useful information can be obtained.

"We do not see how preventing the acquisition of knowledge is a defensible position in an age of reason."

Ms Harrap told BBC News that her group is already aware of Rothamsted's position and arguments and is in the process of replying to an earlier, less detailed letter.

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She also said that the Rothampsted scientists are aware of critiques from science-based opponents of GM technologies such as the group GM Freeze.

Her group does not oppose research, she clarified - but full safety tests should be done before crops are planted outdoors.

She doubted whether "decontamination" would occur on 27 May, given security around the site.

Calling on campaigners publicly not to destroy crops and appealing on the basis of GM crops' environmental credentials is a relatively new tactic for scientists, and was deployed with some success by The Sainsbury Laboratory in Norwich last year.

Protests were held under the banner Take the Spuds Back. But no attempt was made to destroy the site where a trial of a potato modified to resist potato blight, the fungal disease behind the Irish famine of the late 1840s, has entered its third and final year.

Polls continue to indicate a deepset resistance to GM food, in the UK and most of Europe.

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